Light radiation propagating through a nonlinear medium experiences a spectral broadening that can be very substantial (up to two octaves or more) under certain conditions. In early experiments exploiting optical fibers, continuum was formed by broadening and merging separate spectral lines, generated as a result of stimulated Raman scattering (SRS) and four-wave mixing (FWM). Phase matching conditions for the latter were met as a result of multi-mode propagation of light through the fiber. The growing interest to the phenomenon of continuum generation has led to a steady progress in the understanding of the interplay between the different nonlinear processes affecting high power radiation evolution in the optical fiber waveguide.
From a purely practical point of view, progress has also been impressive and has allowed, for example, the generation of supercontinuum radiation with spectral widths in excess of several hundreds of nanometers in microstructured, tapered and highly-nonlinear fibers (HNLF). U.S. Pat. No. 6,775,447, issued to J. W. Nicholson et al. on Aug. 10, 2004 and assigned to the assignee of this application, describes an all-fiber supercontinuum source based on a number of separate sections of HNLF joined together, where each has a different dispersion at the operating wavelength and are joined together so that the dispersion decreases along the length of the HNLF sections. The resultant Nicholson et al. all-fiber source is thus able to generate a continuum spanning more than an octave. While this source is advantageous in all manner of systems where an all-fiber configuration is preferred, the generated supercontinuum bandwidth does not extend into the lower end of the spectrum (i.e., into the visible region) that is considered to be useful in many applications.
Indeed, many of the frequencies that are useful for frequency metrology are in the visible portion of the spectrum, well below the range that has traditionally been used for optical communication applications. Presently, optical standards for frequency metrology at 657 nm (a “visible” wavelength) are now accessed by infrared combs using a frequency doubling technique requiring additional signal paths and nonlinear optical devices. It is preferable to reach this frequency standard directly (with supercontinuum), without the need for the additional nonlinear optical components required to perform frequency doubling. However, the current supercontinuum generated by 1550 nm pump lasers generally do not extend to wavelengths much shorter than 850 nm.
Thus, a need remains in the art for a supercontinuum source capable of generating visible-range radiation without requiring the use of additional nonlinear optical components.